The power supply unit (PSU) provides energy to each individual component in a PC. The factors to consider when choosing a PSU are wattage and efficiency. The wattage states the peak theoretical power output a PSU can provide, but this figure can be overstated. A typical PC will have a peak power draw of 150w. Office PCs with integrated graphics can draw as little as 40w, while gaming PCs with many graphics cards can draw up to 400w. Another figure, PSU efficiency, determines how much electricity is wasted when the PC is turned on. A computer that draws l30w using a power supply that is only 70 per cent efficient will cost more to run than a PC drawing the same power from an SO-percent efficient PSU. For PCs with an add-in graphics card, make sure the PSU has the right connectors for it. High-end graphics card need either one or two 6-pin PCI Express power adapters and some need an S-pin plug. Also research how much power the graphics card manufacturer thinks the PC will need.